|Pos|No|Driver|Constructor|Laps|Time/Retired|Grid|Points|
|---|---|---|---|---|---|---|---|
|1|5|Jim Clark|Lotus-Ford|80|1:59:25.6|1|9|
|2|2|Denny Hulme|Brabham-Repco|80|+ 12.8|4|6|
|3|8|Chris Amon|Ferrari|80|+ 16.6|6|4|
|4|1|Jack Brabham|Brabham-Repco|80|+ 21.8|3|3|
|5|12|Pedro Rodriguez|Cooper-Maserati|79|+ 1 Lap|9|2|
|6|7|John Surtees|Honda|78|+ 2 Laps|7|1|
|7|15|Chris Irwin|BRM|77|+ 3 Laps|13||
|8|20|David Hobbs|BRM|77|+ 3 Laps|14||
|9|14|Alan Rees|Cooper-Maserati|76|+ 4 Laps|15||
|10|18|Guy Ligier|Brabham-Repco|76|+ 4 Laps|21||
|Ret|19|Bob Anderson|Brabham-Climax|67|Engine|17||
|Ret|6|Graham Hill|Lotus-Ford|64|Engine|2||
|Ret|4|Mike Spence|BRM|44|Ignition|11||
|Ret|9|Dan Gurney|Eagle-Weslake|34|Clutch|5||
|Ret|22|Silvio Moser|Cooper-ATS|29|Oil Pressure|20||
|Ret|11|Jochen Rindt|Cooper-Maserati|26|Engine|8||
|Ret|3|Jackie Stewart|BRM|20|Transmission|12||
|Ret|10|Bruce McLaren|Eagle-Weslake|14|Engine|10||
|Ret|17|Jo Siffert|Cooper-Maserati|10|Engine|18||
|Ret|23|Jo Bonnier|Cooper-Maserati|0|Engine|9||
|DNS|16|Piers Courage|BRM|0|Non Starter|||

Question: what is the number of drivers who didn't finish the race due to engine trouble?
Answer: 6